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We have detected the T Tauri star, DO Tauri, in a 0.6'-resolution VLA map of 43.3 GHz (l = 7 mm) continuum emission. Upper limits to the flux densities at 8.4 and 22.5 GHz limit the contribution of free-free emission due to a compact ionized wind to less than 49%.
We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A 0535+26 with the Fermi/Gamma-ray Burst Monitor (GBM) and several ground-based observatories. These new observations are put into the context of the rich historical data (since 1978) and discussed in terms of the neutron-star-Be-disk interaction. The Be circumstellar disk was exceptionally large just before the 2009 December giant outburst, which may explain the origin of the unusual recent X-ray activity of this source. We found a peculiar evolution of the pulse profile during this giant outburst, with the two main components evolving in opposite ways with energy. A hard 30-70 mHz X-ray quasi-periodic oscillation was detected with GBM during this 2009 December giant outburst. It becomes stronger with increasing energy and disappears at energies below 25 keV. In the long term a strong optical/X-ray correlation was found for this system, however in the medium term the Halpha equivalent width and the V-band brightness showed an anti-correlation after 2002 August. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the H showed a strong emission. In late 2010 and before the 2011 February outburst, rapid V/R variations are observed in the strength of the two peaks of the H line. These had a period of 25 days and we suggest the presence of a global one-armed oscillation to explain this scenario. A general pattern might be inferred, where the disk becomes weaker and shows V/R variability beginning 6 months following a giant outburst.
Recent observations of circumstellar disks and their evolutionary timescales are reviewed. It is concluded that disks appear to be a natural outcome of the star-formation process. The disks surrounding young stars initially are massive, with optically thick structures comprised of gas and micron-sized grains. Disk masses are found to range from 0.01 to 0.2 solar masses for solar-type PMS stars, and from 0.01 to 6 solar masses for young, intermediate mass stars. Massive, optically thick accretion disks have accretion rates between 10 exp -8 and 10 exp -6 solar masses/yr for solar type PMS stars and between 10 exp -6 and 10 exp -4 solar masses/yr for intermediate stars. The results suggest that a significant fraction of the mass comprising the star may have passed through a circumstellar accretion disk.
The evolution and appearance of circumstellar disks in star forming regions are influenced strongly by the radiation from either the central star itself or close comparison and nearby hot stars.
Radiative effects strongly hinder the formation of massive stars. A necessary condition for accretion growth of a hydrostatic object up to high masses is the formation of and accretion through a circumstellar disk.
This contribution reports the results of an infrared imaging survey aimed at characterizing the stellar populations associated with the three densest star-forming cores in the Ophiuchus molecular cloud complex. The survey has sufficient sensitivity at J, H, and K (at 5 sigma limits of 16.5, 15.4, and 14.2) to provide a complete census of embedded young stellar objects (YSOs) with masses greater than the hydrogen-burning limit, provided that their ages are less than 3 Myr and that they are obscured by no more than approximately 18 mag of visual extinction. Our data suggest (1) a large fraction (greater than 70%) of the sources located within the cores are still surrounded by circumstellar disks and/or envelopes; and (2) the shape of the initial mass function for masses, M less than 1 solar mass, appears to be consistent with that derived from the solar neighborhood. We also report the results of a deeper imaging survey of centimeter continuum sources (14 sources) in these star-forming cores and in the larger Ophiuchus complex (eight sources). A large fraction (11/14) of the radio sources associated with the cores appear to have infrared excesses diagnostic of circumstellar accretion disks and/or infalling circumstellar envelopes. In these cases, the centimeter continuum radiation most likely diagnoses the ionized component of energetic winds or jets which characterizes YSOs during the disk accretion phase. By contrast, of the eight radio sources located outside dense cores, only two show infrared excesses.
Maser amplification in a Keplerian circumstellar disk seen edge on-the idea put forward by Gordon (1992), Martin-Pintado, & Serabyn (1992), and Thum, Martin-Pintado, & Bachiller (1992) to explain the millimeter hydrogen recombination lines in MWC 349-is further justified and developed here. The double-peaked (vs. possible triple-peaked) form of the observed spectra is explained by the reduced emission from the inner portion of the disk, the portion responsible for the central ('zero velocity') component of a triple-peaked spectrum. Radial gradient of electron density and/or free-free absorption within the disk are identified as the probable causes of this central 'hole' in the disk and of its opacity. We calculate a set of synthetic maser spectra radiated by a homogeneous Keplerian ring seen edge-on and compare them to the H30-alpha observations of Thum et al., averaged over about 1000 days. We used a simple graphical procedure to solve an inverse problem and deduced the probable values of some basic disk and maser parameters. We find that the maser is essentially unsaturated, and that the most probable values of electron temperature. Doppler width of the microturbulence, and electron density, all averaged along the amplification path are, correspondingly, T(sub e) less than or equal to 11,000 K, V(sub micro) less than or equal to 14 km/s, n(sub e) approx. = (3 +/- 2) x 10(exp 7)/cu cm. The model shows that radiation at every frequency within the spectrum arises in a monochromatic 'hot spot.' The maximum optical depth within the 'hot spot' producing radiation at the spectral peak maximum is tau(sub max) approx. = 6 +/- 1; the effective width of the masing ring is approx. = 0.4-0.7 times its outer diameter; the size of the 'hot spot' responsible for the radiation at the spectral peak frequency is approx. = 0.2-0.3 times the distance between the two 'hot spots' corresponding to two peaks. An important derivation of our model is the dynamical mass of the central star, M(sub *) approx. = 26 solar masses (D/1.2 kpc), D being the distance to the star. Prospects for improving the model are discussed.
The wave propagation in a gaseous disk which is self-gravitating and viscous is studied. Waves are generated by an external disturbance and are maintained in motion among forces due to rotation, self-gravity, and pressure. A linear theory is developed for the general case in which all factors are taken into account. The general solution can be applied to the photostellar disk of the solar nebula and the circumstellar disk of a binary in which the effects of self-gravity and pressure are equally important. The theory confirms in great detail that a long wave is excited at one of the Lindblad resonances gravitationally by the external periodic potential, propagates toward the corotation, is reflected before reaching there at the Q-barrier, and finally propagates in the reverse direction toward and past the Lindblad resonance as a short wave.
Gas-poor debris disks represent a fundamentally different class of circumstellar disk than gas-rich protoplanetary disks. Their gas probably originates from the same source as the dust, i.e. planetesimal destruction, but the low gas densities make it difficult to detect. So far, Herschel has detected far-IR gas emission from one debris disk, Beta Pictoris. Here I discuss a well-known debris disk system in the GASPS survey, 49 Ceti. It serves as a case study for modeling low-density gas in optically thin disks. The dust disk appears to be spatially resolved at 70 um. Most interestingly, there appears to be a hint of ClI 158 urn emission at the roughly 2 sigma level. Preliminary modeling suggests that reconciling the sub-mm CO emission from this system with the weak or non-existent far-IR atomic lines may require an unusual chemical composition in the gas of this disk.
Gas-poor debris disks represent a fundamentally different class of circumstellar disk than gas-rich protoplanetary disks. Their gas probably originates from the same source as the dust, i.e. planetesimal destruction, but the low gas densities make it difficult to detect. So far, Herschel has detected far-IR gas emission from only one or two debris disks, Beta Pictoris being one of them. Here we present Herschel GASPS observations of a well-known debris disk system, 49 Ceti. The dust disk is spatially resolved in thermal emission at 70 _m. Most interestingly, weak far-IR gas emission is detected. Preliminary modeling suggests that reconciling the sub-mm CO emission seen from this system with the far-IR gas detection and upper limits requires a low gas-to-dust ratio and possibly an unusual gas composition.
We present far-IR/sub-mm imaging and spectroscopy of 49 Ceti, an unusual circumstellar disk around a nearby young A1V star. The system is famous for showing the dust properties of a debris disk, but the gas properties of a low-mass protoplanetary disk. The data were acquired with the Herschel Space Observatory PACS and SPIRE instruments, largely as part of the “Gas in Protoplanetary Systems” (GASPS) Open Time Key Programme. Disk dust emission is detected in images at 70, 160, 250, 350, and 500 micron; 49 Cet is significantly extended in the 70 micron image, spatially resolving the outer dust disk for the first time. Spectra covering small wavelength ranges centered on eight atomic and molecular emission lines were obtained, including [O i] 63 micron and [C ii] 158 micron. The C ii line was detected at the 5 sigma level—the first detection of atomic emission from the disk. No other emission lines were seen, despite the fact that the Oi line is the brightest one observed in Herschel protoplanetary disk spectra. We present an estimate of the amount of circumstellar atomic gas implied by the C ii emission. The new far-IR/sub-mm data fills in a large gap in the previous spectral energy distribution (SED) of 49 Cet. A simple model of the new SED confirms the two-component structure of the disk: warm inner dust and cold outer dust that produces most of the observed excess. Finally, we discuss preliminary thermochemical modeling of the 49 Cet gas/dust disk and our attempts to match several observational results simultaneously. Although we are not yet successful in doing so, our investigations shed light on the evolutionary status of the 49 Cet gas, which might not be primordial gas but rather secondary gas coming from comets.
We report high-resolution (0.07 arcsec) near-infrared polarized intensity images of the circumstellar disk around the star 2MASS J16042165−2130284 obtained with HiCIAO mounted on the Subaru 8.2 m telescope. We present our H-band data, which clearly exhibit a resolved, face-on disk with a large inner hole for the first time at infrared wavelengths.We detect the centrosymmetric polarization pattern in the circumstellar material as has been observed in other disks. Elliptical fitting gives the semimajor axis, semiminor axis, and position angle (P.A.) of the disk as 63 AU, 62 AU, and −14◦, respectively. The disk is asymmetric, with one dip located at P.A.s of ∼85◦. Our observed disk size agrees well with a previous study of dust and CO emission at submillimeter wavelength with Submillimeter Array. Hence, the near-infrared light is interpreted as scattered light reflected from the inner edge of the disk. Our observations also detect an elongated arc (50 AU) extending over the disk inner hole. It emanates at the inner edge of the western side of the disk, extending inward first, then curving to the northeast. We discuss the possibility that the inner hole, the dip, and the arc that we have observed may be related to the existence of unseen bodies within the disk
We report high-resolution (0.07 arcsec) near-infrared polarized intensity images of the circumstellar disk around the star 2MASS J16042165.2130284 obtained with HiCIAO mounted on the Subaru 8.2 m telescope. We present our H-band data, which clearly exhibit a resolved, face-on disk with a large inner hole for the first time at infrared wavelengths. We detect the centrosymmetric polarization pattern in the circumstellar material as has been observed in other disks. Elliptical fitting gives the semi-major axis, semi-minor axis, and position angle (P.A.) of the disk as 63 AU, 62 AU, and -14deg, respectively. The disk is asymmetric, with one dip located at P.A.s of approx. 85deg. Our observed disk size agrees well with a previous study of dust and CO emission at submillimeter wavelength with Submillimeter Array. Hence, the near-infrared light is interpreted as scattered light reflected from the inner edge of the disk. Our observations also detect an elongated arc (50 AU) extending over the disk inner hole. It emanates at the inner edge of the western side of the disk, extending inward first, then curving to the northeast. We discuss the possibility that the inner hole, the dip, and the arc that we have observed may be related to the existence of unseen bodies within the disk.
A summary of the work completed during the first year of a 5 year program to observationally study the clearing phase of proto-planetary disks is presented. Analysis of archival and current IUE data, together with supporting optical observations has resulted in the identification of 6 new proto-planetary disk systems associated with Herbig Ae/Be stars, the evolutionary precursors of the beta Pictoris system. These systems exhibit large amplitude light and optical color variations which enable us to identify additional systems which are viewed through their circumstellar disks including a number of classical T Tauri stars. On-going IUE observations of Herbig Ae/Be and T Tauri stars with this orientation have enabled us to detect bipolar emission plausibly associated with disk winds. Preliminary circumstellar extinction studies were completed for one star, UX Ori. Intercomparison of the available sample of edge-on systems, with stars ranging from 1-6 solar masses, suggests that the signatures of accreting gas, disk winds, and bipolar flows and the prominence of a dust-scattered light contribution to the integrated light of the system decreases with decreasing IR excess.
By carrying out direct imaging and spectroscopic observations of young, pre-main sequence stars in nearby molecular clouds researchers have begun: to define the frequency with which disks of approximately solar system size and mass form around young stars, and to understand the timescale for disk evolution; to characterize the early radiation (ultraviolet and keV particle) environment of circumstellar disks through study of evolution of stellar winds, wind/disk interactions and the UV and optical emission characteristics of young stars; and to understand the evolution of the solid and gaseous constituents of disks through observations of absorption features in circumstellar gas, broad emission features produced by organic compounds on grain surfaces, and absorption features (e.g., ice) produced in grain mantles. These programs offer the possibilty of relating results from astrophysical studies of the environment to newly-formed stars to the record of planet formation preserved in the solar system. Researchers completed a spectroscopic survey of 30 T Tauri stars with ages ranging from approximately 2 x 10 to the 5th power to 3 x 10 to the 7th power years. From analysis of (OI) and (SII) emission lines, it was concluded that all but two of the stars in our sample are surrounded by optically opaque disks of dimension approximately 50 AU. The two remaining objects show evidence consistent with partial disk clearing (at an age approximately 3 x 10 to the 6th power year). R and I band CFHT observations yielded detections of disks of dimension approximately 100 AU surrounding 5 additional T Tauri stars.
The pre-main sequence T Tauri stars DL Tau and CW Tau were observed with Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) and the Goddard Fabry-Perot Imager at the Apache Point 3.5-m telescope in their coronagraphic modes. The STIS observation of DL Tau shows a circumstellar disk with a dark ring, and the jet with structure along the length, and a faint counter-jet. On CW Tau, STIS shows a jet with a bright blob about 4 arcsec away, and the Fabry-Perot shows the jet extended in both directions in [SII] to 60 and 90 arcsec from the star.
The growth of global gravitational instabilities in young stellar objects (YSOs) with associated circumstellar disks is studied. The possibility that the accretion ultimately owes its origin to the growth of spiral gravitational instabilities is explored. The results indicate that YSO disks will be unstable to the growth of eccentric distortions which have growth rates comparable to the orbital frequency at the outer edge of the disk. Thus, the distortions grow on nearly a dynamical time scale. Perturbations with m = 1 force the star to move from the center of mass and thereby transfer angular momentum to the stellar orbit. Depending on whether or not an axisymmetric stability parameter Q barrier exists near the corotation radius of the disturbance, this coupling may lead to mass accretion or to the formation of a binary companion from the disk, or both.
"High-resolution NIR spectroscopic observations of warm molecular gas emission from young circumstellar disks allow us to constrain the temperature and composition of material in the inner planet-forming region. By combining advanced data reduction algorithms with accurate modeling of the terrestrial atmospheric spectrum and a novel double-differencing data analysis technique, we have achieved very high-contrast measurements (S/N approx. 500-1000) of molecular emission at 3 microns. In disks around low-mass stars, we have achieved the first detections of emission from HCN and C2H2 at near-infrared wavelengths from several bright T Tauri stars using the CRIRES spectrograph on the Very Large Telescope and NIRSPEC spectrograph on the Keck Telescope. We spectrally resolve the line shape, showing that the emission has both a Keplerian and non-Keplerian component as observed previously for CO emission. We used a simplified single-temperature local thermal equilibrium (LTE) slab model with a Gaussian line profile to make line identifications and determine a best-fit temperature and initial abundance ratios, and we then compared these values with constraints derived from a detailed disk radiative transfer model assuming LTE excitation but utilizing a realistic temperature and density structure. Abundance ratios from both sets of models are consistent with each other and consistent with expected values from theoretical chemical models, and analysis of the line shapes suggests that the molecular emission originates from within a narrow region in the inner disk (R < 1 AU)."